Many tools in a semiconductor manufacturing facility process wafers in a high vacuum environment. Because the wafers travel from one tool to another at atmosphere, each tool has a way of loading the wafers into a transitional vacuum chamber (load lock) to remove the air pressure before they are handled by the machine into its high vacuum environment. Conversely, this same load lock is typically used to return the wafers from high vacuum environment back to atmosphere for removal after being processed.
While travelling into and out of the tool, the wafers will go through many processes that could potentially stir up and deposit particles onto their surface. For all high vacuum processing tools, a pressure burst which occurs while opening the load lock door after a vent may be a leading cause of particulate contamination. This contamination can destroy the semiconductor devices on top of the wafers and, as their feature sizes continue to shrink, this particle effect on device yield is magnified.
As the critical dimensions of microelectronic devices continue to shrink on silicon wafers, there have been ever increasing efforts to reduce the amount of airborne particles on the wafers as they are handled between processes on semiconductor manufacturing equipment. As stated above, it has been well known that a major potential source of particles on wafers is during the transition between the high vacuum of the semiconductor tool in which they are processed back to atmospheric pressure prior to being removed from the machine. When the load lock door opens at the end of this transition, a significant pressure burst can dislodge dust particles from nearby surfaces. These dust particles can deposit on the wafers as they are being removed.
A load lock is typically vented with nitrogen. If, before opening the door, the N2 pressure in the load lock is greater than the outside air, a positive pressure burst will result. If this burst is large enough, the burst flow can lift dust particles from nearby surfaces around the door and inside the load lock. Then, as the wafers are handled in and out of the load lock, the particles can settle on top of the wafers. A pressure burst in the opposite direction may be created if the door valve is opened when the N2 pressure is significantly less than the outside air pressure. Studies have shown that that this scenario may lead to worse contamination as outside air rushes past the door's o-rings, lifting up dust particles and depositing them on the wafers and load lock surfaces inside.
In general, it is desirable to keep the turbulence of N2 and air to a minimum when the door is opened. Theoretically, if the pressure of the inside and outside air was equal, there would be no turbulence and the risk of particle contamination would be minimized. Practically, with the accuracy of sensors and valve response delays, it can be difficult to get the pressure difference to near zero before opening the door. Also, adding long pressure stabilization delay times is to be avoided in the high-throughput environment of the semiconductor fabrication, where wafers are introduced, processed and then removed from the machine as fast as possible.
In summary, it would be beneficial if there existed a load lock system and method to minimize pressure bursts while operating at a high throughput. Further, it would be advantageous if the system and method adapted to changes in system's dynamic vent response.